A semiconductor optical amplifier (SOA) disposed on a GaAs substrate and provided with an active layer including a quantum dot has excellent characteristics, e.g., a high temperature characteristic and a high saturation optical output characteristic.
However, the quantum dot has a flat shape, and a compressive strain resulting from a lattice mismatch is inherent therein. Consequently, the gain of the quantum dot SOA exhibits large polarization dependence. That is, regarding the quantum dot SOA, the gain with respect to the light in a transverse electric (TE) mode is larger than the gain with respect to the light in a transverse magnetic (TM) mode. Therefore, if the quantum dot SOA is used, the light in the TE mode is amplified to a great extent as compared with the light in the TM mode.
There are various technologies to improve the polarization dependence.
For example, there is a technology to improve the polarization dependence by quantum-mechanically coupling a plurality of quantum dots through repetition of operations in which InAs quantum dots are formed on a barrier layer disposed on a GaAs substrate and the InAs quantum dots are buried with the barrier layer so as to form a quantum dot layer. In this regard, as for the barrier layer, GaAs, AlGaAs, InGaAs, InGaAsP, InAlGaAs, InAlGaP, and the like are used. Hereafter this technology is referred to as a first technology.
For example, there is a technology to improve the polarization dependence by forming an InAs quantum dot on a barrier layer disposed above an InP substrate and forming a side barrier layer having an elongation strain in such a way as to come into contact with the side surface of the InAs quantum dot. In this regard, as for the barrier layer and the side barrier layer, group III-V compound semiconductor materials, e.g., InGaAsP, InGaAs, InAlGaAs, InAlGaP, and GaInNAs, containing In and Ga are used. Hereafter this technology is referred to as a second technology.
For example, there is a technology to realize a polarization-insensitive gain in a 1.55 μm band by using an InGaAsP layer with an elongation strain as a side barrier layer in a SOA which is disposed above an InP substrate and which includes a columnar quantum dot. Hereafter this technology is referred to as a third technology.
In the above-described first technology, it is necessary that the number of lamination of quantum dot layers is increased in order to satisfactorily increase the gain with respect to the light in the TM mode and realize the polarization-insensitive gain. However, it is not easy to increase the number of lamination of quantum dot layers. Consequently, it is not easy to obtain the polarization-insensitive gain.
Furthermore, the above-described second technology and the above-described third technology are technologies in the case where the InAs quantum dot is formed on the barrier layer disposed on the InP substrate and are not technologies in the case where the InAs-containing quantum dot on the GaAs-containing barrier layer is disposed on the GaAs substrate. Consequently, even when the above-described second technology and the above-described third technology are applied to the case where the InAs-containing quantum dot is formed on the GaAs-containing barrier layer, it is difficult to obtain a polarization-insensitive gain.
As for the related art, there are literatures as described below.
Japanese Unexamined Patent Application Publication No. 2004-111710, Japanese Unexamined Patent Application Publication No. 2006-245373, N. Yasuoka et al., “1.55-μm Polarization-Insensitive Quantum Dot Semiconductor Optical Amplifier,” ECOC 2008, 21-25 Sep. 2008, Brussels, Belgium, Th.1.C.1, Vol. 4-17, 18.